The present invention relates to electrical contact structures particularly usable in low power circuits, expecially used as an interface between electromechanical devices and electronic circuits.
Owing to the higher and higher diffusion and to the cheaper and cheaper costs of semiconductor devices and systems and, in particular, of microprocessors in fields once almost completely covered by the electromechanics, it is more and more present the problem of interfacing electromechanical devices with electronic devices and, while the interfacing from electronic devices to electromechanical devices is usefully obtained through the use of amplifiers (buffers) providing high enough power to directly drive the electromechanical devices, the reverse interface from electromechanical devices to electronic devices suffers from easily predictable problems.
It should seem obvious in producing digital signals to be inputted in electronic devices, to use the theoretical digital features of a pair of electromechanical contacts whose opening and closure could exactly correspond to logical states "0" and "1", or vice versa according to the case. However, the electromechanical contacts, when used in particularly low power circuits, as for example with supply voltages lower than 10 volts and currents lower than 1 milliampere, can introduce problems in the faithful correspondence between opening and closure of said contacts and the associated logical states. These problems occur chiefly from the fact, that the surfaces of electromechanical contacts are never completely free from scales (specifically metal oxide and sulfide scales) or at any rate from extraneous non conductive matters which can unduly raise the contact resistance simulating a contact opening instead of a contact closure.
In circuits, having power higher than that of the electronic circuits, such as those used in the electromechanical field (as for example circuits containing low inductive loads such as microrelays), such a situation tends to disappear because, owing to the involved voltages and currents which are easily formed from rebounds after the closure of a contact pair.
To obviate the problem of poor correspondence between opening and closure states of contacts and associated logical states, there have been offered many different approaches.
A first approach consists in coating contact pads with noble metals (gold) with substantial cost raises and poor efficiency with respect to pollution from extraneous matters.
A second approach consists in using refractory metal coated contacts on resilient arms enclosed in electrically insulating bulbs (such as glass) in which an inert gas is enclosed (they are the well known "dry reed" contacts) assuring an excellent correspondence between opening and closure states and logical states, but they have the drawback of high cost, limited useful embodibility in just some specifical relays, substantial current limitation not permitting loads also just a little higher than the rated ones, under penalty of contact and contact arm over heating, and too high, sensitiveness to electromagnetic interferences and to accelerations or generally to mechanical shocks.
A third approach consists in employing usual contacts in free air whose number is multiplied to increase the number of connecting points, such as two movable contact pairs on two movable bridges, in parallel or two movable contact pairs on crossed bridges.
This approach, while is not very simple and inexpensive, does not obviate the problem of the poor correspondence between opening and closure states and logical states, because it can always happen that a heavy enough scale make the contact substantially insulated.
A fourth approach provides movable contacts mounted in pairs on a conductive bridge with the bridge providing, further to the approaching movement to a fixed contact pair, also a lateral or cross movement, once the contacts are in touch, for wiping and scraping the movable contacts against said fixed contacts in order to clean the mutually faced surfaces of said contacts.
This system can work for what regards the contact surface cleaning from metal oxides and sulfides, but has the drawback that, having the conductive bridge to slide on insulating material portions of a driving mechanism, wears away said insulating material, producing contact soiling by said material.
A further approach consists in using movable contact pairs, having substantially hemispherical shape, fastened to a conductive bridge, pressing on a fixed contact pair indexed according to a sloping plane with respect to the tangential point of said movable contacts, so that the movable contacts are compelled to accomplish a very limited rotation when engage the fixed contacts, but having to concurrently slide on insulating material portions, wear them and produce soiling of the contacts by the insulating material itself.
This drawback of contact soiling by worn insulating material could be avoided using movable contact arms resiliently strained, which, however have the drawback of poor reliability because, if an overcurrent flows through the resilient arms, it may happen that they are annealed by overheating due to overcurrent, losing their resilience and thus the feature of making the movable contacts wiping or rubbing against the fixed contacts.